Microsoft provides source access to some customers via its Code Center Premium (CCP) program. Not many people are aware that Visual Studio 2005 has integrated CCP support.
It is true! While debugging applications in VS you can download sources for system files from the code premium website provided you have CCP access. The prompting for the CCP card pin is handled in-session. The same license agreements apply.
This link gives you complete instructions on how to go about enabling and using this with VS 2005 - http://msdn2.microsoft.com/en-us/library/ms164729.aspx
Note that the CCP program is independent of Visual Studio and we are not involved in providing CCP source access. If you are interested in the program or have any complaints accessing the files you would need to contact the CCP team - http://www.microsoft.com/resources/sharedsource/ccp/premium.mspx